In order to prevent agricultural products, especially bulk foods such as grains, from becoming unusable on ships, a treatment with pesticides is necessary.
To do so, the bulk material is sprayed with a liquid insecticide, but the agent acts only at the surface and kills only the pests that come in contact with the surface. Such agents do not destroy the developing stages from egg to pupa. Another disadvantage is that the liquid insecticides and their residues cannot be removed from the bulk material subsequently.
In another process, the bulk material is fumigated with methyl bromide or hydrogen cyanide. However, a disadvantage of the treatment with methyl bromide is that almost all the gaseous methyl bromide collects near the bottom due to its high specific gravity and thus is not available for the desired purpose in a sufficient concentration in the entire volume of stored bulk food. In addition, there is the problem of the presumed carcinogenic effect of gaseous methyl bromide which raises questions regarding unlimited international use of this preparation in the future.
Similar use of hydrogen cyanide also raises the objection of its great power of adhesion to the surface of the bulk material and thus inadequate depth of penetration.
Another method of eliminating pests is fumigation with a preparation that releases phosphine and has been performed in the past by either
introducing the preparation in the form of pressed tablets into the bulk material by means of a probe, or
adding the preparation in the form of pellets or tablets by means of a metering device into the stream of bulk material flowing through the loading hatch, or
adding the preparation to the bulk material in the form of bags which contain the preparation which releases phosphine by either throwing the bags, pulling them in with special tools or placing them on the surface of the bulk material.
The process for introducing pellets or tablets by means of a probe is very labor-intensive, and the depth of penetration is limited to a maximum of 10 meters because the probes must be introduced into the bulk material with physical force. The depth of the bulk packing on a ship may reach up to 25 meters, however. A uniform distribution of the gas is possible only after a long period of time, so destruction of the pests is not assured in this way. Furthermore, the residues remain in the bulk material.
Introduction of pellets through a metering device is connected with disadvantages in those cases when the bulk material is not introduced continuously through a conveyor device, but instead by means of a bagger, because the preparation and thus the gas are not uniformly distributed in the bulk material.
Use of bags also leads to an uneven distribution of the gas because the bags cannot be introduced at random into the bulk material. In order to avoid these disadvantages, the bags must be added to the feed stream. Since the bags must be removed from the bulk material, a screening or sorting step is also necessary.
Recently, there have been attempts to eliminate the disadvantages of the processes known in the past by placing preparations in the form of bags and chains of bags that release phosphine on or directly beneath the surface of the bulk food material in order to facilitate introduction and subsequent removal.
However, this method has a disadvantage that, depending on the depth of the stored bulk material, it may take several days to achieve the required gas concentration in the entire bulk material. Furthermore, the concentration which develops between the surface of the bulk material and the cover, i.e., the loading depth, may become so high that it greatly exceeds the lower explosion limit.
Furthermore, when using the above-mentioned preparations in so-called "in-transit shipboard fumigation," there is the risk that quantities of gas that are harmful for the crew might enter personal quarters intended for the crew, recreation rooms, mess halls, bridges, machine room, etc., and thus represent a latent hazard for the crew. Cases are also known where fires erupted due to shifting of the chains of bags and local accumulations, posing a direct threat to the ship and crew.
German Patent (OLS) No. 2,945,334 describes a process for treatment of agricultural products stored in a container whereby a product is introduced into a storage vessel, air is supplied to the storage vessel, a gaseous chemical agent which has a minimal sorption with respect to the product is introduced into the storage vessel and air and the chemical agent are circulated through the product within the vessel at a low velocity of flow by means of the air supplied until a uniform distribution of the chemical agent in the entire product is achieved. The rate at which the air is circulated is lower than about 0.005 m.sup.3 /min.m.sup.3 and is preferably between 0.0012 m.sup.3 /min.m.sup.3 and 0.0006 m.sup.3 /min.m.sup.3. The preferred chemical agent is phosphine, which is produced from aluminum phosphide or magnesium phosphide.
The equipment described in this publication and needed for carrying out the process is not suitable for ships due to its design, but instead is suitable only for stationary silos. The equipment necessary for carrying out this process is thus considerably more complex and is suitable only for a stationary installation in a silo.